The basic configuration of a tilting mirror based VOA is shown schematically in FIG. 1a and comprises of a lens 1, a mirror 2, an input optical waveguide, e.g. input fiber 3, and an output optical waveguide, e.g. output fiber 4. Input light 5, including a plurality of wavelength components, e.g. red and blue wavelengths, from the input fiber 3 is collimated by the lens 1, and reflected by the mirror 2 forming a reflected beam 6a. The reflected beam 6a is then re-focused by the lens 1 into the output fiber 4. When the mirror 2 is set at an angle at which the reflected beam 6a is focused at the core 8 of the output fiber 4, i.e. an optimum position, the insertion loss (IL) of this coupling system is at a minimum (See FIGS. 1a and 1b). When the mirror 2 is tilted, as in FIGS. 2a and 2b, the reflected beam, identified by 6b, becomes off-set from the core 8 of the output fiber 4 causing an increase in IL, and therefore attenuation. The amount of IL, and therefore the amount of attenuation will be determined by the amount of off-set from the core 8, i.e. the tilt angle of the mirror 2 from the optimum position, shown in FIG. 1a. However, this configuration has an intrinsic loss called wavelength dependent loss (WDL) and is a function of attenuation and is related to differences in mode field diameters (MFD) of the different wavelength components. More specifically, the MFD or mode spot size of an optical fiber is a function of wavelength, and in an optical fiber, light with shorter wavelength components (blue light) is more tightly confined in the core of the optical fiber than light with longer wavelength components (red light), i.e. as illustrated in FIGS. 1b and 2b, the spot size for the blue light 21 is smaller than the spot size of the red light 22. The problem with the prior art VOAs, as illustrated in FIG. 2b, is that when the mirror 2 is tilted, the center of the reflected light beam 6b is off-set from the second fiber core 8 by the same amount for all wavelengths and the efficiency for coupling light into the output fiber 4 depends on the degree of overlap of the light spot size with the fiber core 8. For light beams having the same off-set, the shorter wavelength components have a smaller light spot size and will experience more coupling loss than the longer wavelength components that have the larger spot size because the larger spot size will simply over lap the core 8 more than the smaller spot size.
FIG. 1b illustrates a cross-sectional view of the output fiber 4 having a core 8 in which a blue light spot 21 and a red light spot 22 are concentric to the core 8 when the VOA is in the minimal attenuation state; and FIG. 2b illustrates a typical attenuation state with the non-concentric positions of the light spots, and with the blue light spot 21 overlapping the core at a smaller percentage than the red light spot 22 thereby causing greater IL loss for the blue light which is called wavelength dependant loss, WDL.
It's well known that IL can be expressed as the function of beam offset within the fiber and MFD (Mode Field Diameter) as shown in the following equation (1):
                    IL        =                  4.34          ·                                    (                              x                ω                            )                        2                                              (        1        )            Where x is the offset of the focused beam from core of the fiber, and ω is the half MFD. The ω in equation (1) is a function of wavelength λ, and it is in a linear relation with the wavelength when the wavelength is in a small range (e.g., C band or L band) and can be expressed by equation (2) below:ω=a+b·λ  (2)where a is a constant, and b is the linear chromatic dispersion coefficient. WDL can be then calculated using equation (3):
                    WDL        =                              -            2                    ·          b          ·                      IL            ω                    ·          Δλ                                    (        3        )            
For Corning SMF28 fiber, b is approximately equal to 3.11 in the C band and L band ranges. It is clear from (3) that when IL increases, WDL increases also, and a longer wavelength light, e.g. red light has less IL than a shorter wavelength light, e.g. blue light.
In the prior art there are a number of VOA systems that attempt to deal with WDL. One of these systems is disclosed in U.S. Patent Publication No. 2004/0008967, which tries to solve the problems of WDL and PDL (polarization dependent loss) by using a collimator comprised of various optical components including a ferrule holding at least two waveguides and a lens. The ferrule and the lens are selected such that the plane containing the end of the ferrule and the ends of the waveguides are not parallel to the facing end of the lens, so that it is possible to determine positions and axial orientations of the ferrule with respect to the lens which result in minimal WDL. However, there is no consideration given to MFD and the positional adjustment that could be used to offset WDL.
The VOA system disclosed in U.S. Pat. No. 6,915,061 utilizes a wedge that is placed between two lenses in order to help in focusing and reduce the size of the micromirror.
U.S. Pat. No. 6,782,153 discloses a device that has a filter, whereby the device performs two or more of the functions of wavelength division multiplexing or demultiplexing, attenuation, switching, filtering and tapping functions. The filter element preferably has a wedged cross-section in order to prevent an etalon caused by the two faces of the filter element, but there is no concern with reducing WDL.
U.S. Patent Publication No. 2004/0136680 discloses a VOA that has a semitransparent refractive mode shutter comprising a silicon shutter shaped as a wedge that provides variable tilt for an output beam, and therefore variable attenuation. Other parameters such as PDL, optical reflection losses (ORL) and WDL are also a function of the shutter geometry, but there is no concern for reducing WDL.
U.S. Patent Publication No. 2002/0061179 has a VOA employing shutters with a V-shaped notch that has a form adapted for reducing the dependency of optical attenuation rate on wavelength. The system shutters light so that the dependency of optical attenuation rate due to changes in mode field diameters caused by different wavelengths is reduced.
U.S. Pat. No. 7,034,979 discloses a VOA that uses a crystal wedge in polarization modulation wherein the crystal wedge is used to spatially recombine polarized beams, but there is no consideration of WDL.